The present invention relates to novel inks, more particularly, to aqueous black inks which can be transferred in droplets onto a print carrier, and to processes for their production and use.
German Auslegeschrift No. 2,164,614, and DOS 2,210,512, incorporated herein by reference, for example, disclose ink ejection processes in which temporary pressures are producted in an ejection or writing head leading to the discharge of ink from capillary nozzles and to interruption of this ink flow to form droplets. In this process, the ink separates in droplets from the nozzle outlet area.
In order for an ink to be suitable for such ejection processes, certain parameters must be present. Thus, clean type characters require, among other things, uniformly sized ink drops which are to be applied in a grid pattern onto the print carrier. The production of ink drops of uniform size depends, on the one hand, on the geometry of the nozzle and the configuration of the break-off plane and, on the other hand, on ink having satisfactory wetting behavior within and/or outside the nozzles. To stabilize the ink meniscus in the nozzles and for the droplet formation, the ink requires a surface tension as high as possible.
The supply of ink into the nozzles of the ejection head, for example, from an ink reservoir, is effected, in part, through capillaries, often without pressure for the replenishment of the ink. In order to attain a flow of ink in sufficient amounts to achieve a high droplet sequence, and thus a high ejection speed, the ink must also have a low viscosity, e.g., about 1.70 centipoises.
The selection of suitable inks is limited, on the one hand, by its drying or thickening behavior. When the ejection system is at rest, ink components must not crystallize out in the area of the ejection head after long periods of inactivity. Similarly, during these rest periods, the ink ingredients must not colloidly segragate from the ink to form deposits. On the other hand, no surface film must form at the nozzle opening as a result of the evaporation of water.
To render ink ejection processes more economical, the writing head can consist of individual and nickel-plated steel laminae which are bonded together by the diffusion welding method into a composite writing head. The ink should not attack steel (e.g. "Nirosta," a stainless steel with a high chromium content) or, especially, the intermediate diffusion layers. The danger of pitting exists due to halogenides (Cl, Br, I) in the ink. The ink, once filled into the writing head, must exhibit a satisfactory shelf life, it must not react with the coating material covering the nozzles, and it must not be subject to attack by microorganisms. Various inks were found to be prone to frothing due to dye particle accumulations at the interfaces of writing head material and ink, and air and ink, leading to the failure of individual systems of the writing head.
Satisfactory legibility of type characters and symbols on the print carrier requires not only uniform droplets but also uniform droplet deposition on the print carrier. Ink spreading on the print carrier, especially in the fiber direction, substantially impairs the printed image which cannot be corrected by an increase in the fineness of the grid pattern. Additional parameters for a satisfactory ink are its adaptability to the selected print carrier and its lightfastness, resistance to smearing and resistance to forgery.
German Unexamined Laid-Open Application DOS 2,132,324 discloses an aqueous writing ink fed under slight hydrostatic pressure to the writing pen of a writing implement. The writing ink contains, in addition to water as primary ingredient, smaller amounts of a water-soluble lower alkyl ether of ethylene or diethylene glycol, water-soluble dyes, and a water-soluble resin from the group of polyvinylpyrrolidone and polyethylene oxides. An amine is employed to adjust the pH value and neutralize the acidic additives. Due to the presence of the alkyl-ether compound, the surface tension of the ink is so greatly reduced, that the formation of droplets during the ejection of the ink from writing heads of the type mentioned above is impossible. Moreover, during periods of long inactivity, a tough film forms in the outlet aperture of each nozzle, instead of the required liquid meniscus, and this film cannot even be penetrated by the impulses from a compression system with a ceramic vibrator. This ink is unsuitable for use in ink ejection printing processes.
Inks are disclosed in DOS No. 2,160,475 for use in jet printing processes. One or more water-soluble dyes are utilized therein along with a moisturizing system based on a mixture of lower alkoxy triglycols which does not adversely affect the viscosity in spite of water loss from the ink. However, the alkoxy glycols lead to a strong reduction of the surface tension and, as in case of the above-discussed ink, carbon dioxide is absorbed to a large extent from the atmosphere, whereby the pH value of the ink is lowered, which, when using organic dyes, can result in the precipitation of deposits.
U.S. Pat. No. 3,705,045 describes the use and suitability of triarylmethane dyes in inks. Additives to the ink, such as glycols, are employed for avoiding crystallization deposits, based on the assumption that mixtures of alkaline and acidic dyes produce precipitates which can be dissolved only with difficulty. The disclosed process is restricted to the described specific classes of dyes and very accurate mixing ratios must be maintained.
U.S. Pat. No. 1,787,233 describes the use of citric acid as an ink additive, primarily to lower the freezing point of the mixture. Because the pH of the ink ranges at values aroung 1, the ink is unsuitable for use in writing heads, especially metal writing heads.
It is further known to use moisturing agents, e.g., glycerin, in commercially available aqueous inks to retard drying. However, this additive increases the viscosity and the color intensity of the ink is adequate at best. The pH value of such inks is 1.8, as compared with a pH value of about 6, which is the advantageous value for a writing method described hereinabove.
In order to avoid drying, moisturizing agents, such as glycerin, for example, are conventionally added to commericially available water-based inks. Such additives, however, increase viscosity. The color intensity of such inks is adequate at most and their pH is about 1.8, which is substantially below the range suitable for the above-described ejection printing processes.
In addition, German Patent No. 2,511,902, corresponding to U.S. Ser. No. 667,378, filed Mar. 16, 1978, now U.S. Pat. No. 4,101,329, an aqueous ink for an ink ejection process, particularly a blue ink. The ink comprises a water-soluble triphenyl methane dye and a buffer mixture of an organic hydroxy, containing carboxylic acid and a water-soluble amine.